Project Summary/Abstract. One of the great challenges in basic and clinical neuroscience is to understand fundamental organizing principles during nervous system wiring and to identify cell types and responses to molecular signals that sculpt the developing vertebrate brain. The goal of the proposed studies is to further explore molecular and cellular mechanisms of visual circuit formation, particularly those that influence postsynaptic neuronal morphology and connectivity during retinotectal wiring. Here, we will test the hypothesis that a subset of molecular signals interact to control dendritic arbor structure and connectivity in the intact, developing brain, where age-dependent responses influence both early growth and maintenance of a functional dendritic arbor. Over the years, the work of our laboratory has focused on the cellular and molecular mechanisms that control retinotectal circuit development by focusing on the role that the neurotrophic factor BDNF and the classical axon guidance molecules netrin-1, play during wiring events that follow successful retinal ganglion cell (RGC) axon pathfinding. The proposed studies build on the novel and important finding that netrin-1 plays a differential, but key role in the early development of neuronal morphology and connectivity of central neurons, particularly of those neurons that receive input from RGCs. Specifically, netrin influences the directionality of dendrite growth and the branching and pruning of optic tectal neuron dendritic arbors in a manner that is quite different from netrin regulation of RGC axon branching. In the proposed studies real-time imaging of neurons in developing Xenopus laevis embryos and state-of-the-art gain- and loss-of-function approaches will be used chronically and acutely manipulate netrin receptor signaling to demonstrate that regulated expression of netrin and its specific receptors shape postsynaptic connectivity in the retinotectal system by inducing both localized and rapid responses on developing dendrites of central neurons. These studies will also examine the hypothesis that developmental and activity related changes in netrin function impact visual circuit formation and the functional maturation of the visual circuit. Furthermore, our studies will establish that the cell adhesion molecule Down syndrome cell adhesion molecule (DSCAM) is required for dendrite self-avoidance and collaborates with netrin to establish the patterning of tectal neuron dendritic arbors. By imaging responses of genetically modified individual neurons in the intact, developing brain, our studies will advance our understanding of the early mechanisms that sculpt dendrites and synapses in the vertebrate brain, and further our knowledge of the organizing principles that shape functional connectivity in the central nervous system. Elucidating early molecular mechanisms of circuit development that differentially impact pre- and postsynaptic connectivity has broader implications for understanding human neurodevelopmental disorders in which altered brain function is associated with deficits in early connectivity, such as mental retardation and autism.